Update: I bought the Steam version of Avernum: Escape From the Pit and it has issues with lag. However, it is the OpenGL version that is problematic, as I found when I downloaded the demo and tried the DirectX exe included with it.
Unfortunately no such exe is included with Steam's version, which causes problems.
So if anyone out there is frustrated by Steam's version of Avernum, I would suggest downloading the demo, trying the DirectX exe in the install directory, and seeing if that fixes your problems. If so, contact Spiderweb's support and let them know your Steam name and request an unlock key.